1. Field of the Invention
This invention pertains generally to video encoding and decoding, and more particularly to enhanced signaling improvements to Sample Adaptive Offsets (SAO) within high-efficiency video coding (HEVC) systems.
2. Description of Related Art
A significant and ongoing demand continues for high efficiency video coding (HEVC) toward more optimal and scalable video compression. The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T and ISO/IEC MPEG continues enhancing a high efficiency video coding (HEVC) standard.
In HEVC, the frames are divided into coding tree blocks (CTBs) that can be further subdivided into coding units (CUs), such as using quadtree segmentation. A coding unit (CU) may have variable sizes depending on video content toward achieving a desired coding efficiency. CUs typically include a luminance component Y and two chroma components, U and V. The size of U and V components relate to the number of samples, and can be the same or different from that of the Y component, as depends upon the video sampling format. The CUs can be further subdivided into prediction unit (PU) partitions during intra-prediction and inter-prediction as well as transform units (TUs) defined for transform and quantization. Transform units (TU) generally refer to a block of residual data to which a transform is applied when generating transform coefficients.
HEVC introduces new elements over current video coding systems, such as H.264/AVC, and similar codecs. For example, although HEVC still involves motion compensated inter predictions, transforms, and entropy coding, it utilizes either arithmetic coding or variable length coding. In addition, following the deblocking filter is a sample adaptive offset (SAO) filter. The SAO filter unit operates once for each pixel in the CTB. For each CTB, a filter type and offset values are coded in the bitstream. There are two types of filters, specifically, edge and band filters. The band of a given sample is simply the upper 5 bits of its value. Then a band index is transmitted, along with the four offsets, that identifies four adjacent bands. So if the band index is 4, it means bands 4, 5, 6 and 7. If a pixel falls into one of these bands, the corresponding offset is added to it. In an edge filter an edge mode is transmitted (e.g., 0, 90, 45, 135 degrees). The offsets and filter modes are picked by the encoder toward making the CTB more closely match the source image.
The SAO filter considers the entire frame as a hierarchical quadtree. Quadrants in this quadtree are activated by the SAO transmitting syntax values which each represent an intensity band of pixel values referred to as band offset (BO) or the difference compared to neighboring pixel intensities referred to as edge offset (EO). For each type in SAO (BO and EO), transmitted offset values referred to as SAO offset are added to the corresponding pixels.
For example, HEVC working document 7 (WD 7) provides SAO on/off, 4 Edge Offset (EO) classes, and one Band Offset (BO). These SAO types are signaled to the decoder using a Unary code with two contexts for context-adaptive binary arithmetic coding (CABAC) coding, one on the first bin and one for the rest of the bins. In addition, for the BO type, band position is transmitted using FL codes. Context coded bins and by-pass coded bins are interleaved, which limit the CABAC engine throughput.
The present invention provides forms of type decoupling which improve the efficiency of SAO signaling within a video coding system.